1. Field of the Invention
The present invention relates to a method of reducing NOx emissions from coal fired furnaces. More particularly it relates to the reduction of NOx emissions from the combustion of pulverized coal having volatile matter which is low in heat content.
2. Description of the Prior Art
Nitric oxide (NO) is an air pollutant. In many areas of the United States, as well as other countries, methods are sought to reduce its concentration level in flue gases emitted from coal-fired boilers. In the combustion of fuel in the boilers, one problem is the production of nitric oxide due to oxidation of both fuel-bound nitrogen and nitrogen entering with the combustion air.
A portion of the nitric oxide produced by a burner oxidizes to form nitrogen dioxide (NO.sub.2) downstream of the combustion process, as well as in the atmosphere. Consequently, production of nitric oxide at the burner results in both NO and NO.sub.2, commonly called NOx, being emitted into the atmosphere.
This invention addresses two concerns of pulverized coal combustion:
(1) further reduction of NOx produced by commercially available low-NOx burners, and
(2) maintenance of a stable flame on these burners so that coals having a wider range heat of contents can be successfully and stably burned.
In pulverized coal combustion as practiced in boilers, kilns, and other combustion devices, the pulverized coal is generally conveyed to the burners by the "primary" air stream. The primary air in many cases is preheated, dries the coal and carries the coal out of the pulverizer. The ratio of primary air to coal is typically between 1 and 3 on a weight basis to best accomplish these functions.
As the coal and primary air stream enters the furnace via the burner, heat from downstream combustion is transported by recirculated gases and radiation back to the incoming coal particles causing them to heat and devolatilize. The volatiles that are released from the coal particles mix with the primary air, and the temperature and fuel/air ratio of the mixture eventually become sufficient for ignition to occur. The ignition stability of the burner thus depends mainly upon this process of heat transfer from the primary flame zone, devolatilization of the coal, and ignition of the coal volatiles-primary air mixture and/or the solid coal.
As mentioned above, the region immediately following the ignition zone of the burner, in which the coal devolatilization is completed and the volatiles are burned, is generally termed the "primary flame" zone. In this zone, the bulk of the combustion air, i.e., the "secondary" air which is admitted separately from the primary air, mixes with the fuel and burns. The primary flame zone is followed by a char burnout zone in which the devolatilized coal particles are burned in an atmosphere of typically 15% to 25% excess air (i.e., 3% to 5% O.sub.2).
Recent experience has shown that coals in which the volatile matter is low in heat content are more difficult to ignite and burn in flames. Such coals do not release heat rapidly enough to establish a stable ignition zone. Another phenomenon that occurs with coals that are only marginally adequate in quality and extent of volatiles content is that of flame lift-off. In this case, a quasi-stable ignition zone is established, but at a relatively large distance from the burner due to the longer coal particle residence time required to produce the heat required for ignition.
In testing the ignition stabilities of a range of coals to deliberately explore the effect of volatiles content on ignition stability, it was learned that coals with less than approximately 3400 Btu/lb volatile heat content, HHV.sub.vol, tend to have unstable ignition characteristics. The parameter HHV.sub.vol can be calculated from: EQU HHV.sub.vol =HHV.sub.coal -(1-VM)HHV.sub.char ( 1)
where
HHV.sub.vol =higher heating value of volatiles, Btu/lb coal PA1 HHV.sub.coal =higher heating value of coal, Btu/lb coal PA1 VM=volatile matter in coal, percentage PA1 HHV.sub.char =higher heating value of char, Btu/lb.
Formation of NO occurs in both the primary flame zone and the char burnout zone. In the primary flame, NO forms primarily from oxidation of volatilized organic nitrogen compounds. In the char burnout zone, NO forms primarily by oxidation of organic nitrogen compounds in the char, and to a minor extent by oxidation of nitrogen in the air. These three NO formation mechanisms can be summarized as follows: ##STR1##
Work by Pohl and Sarofim, reported in "Devolatilization and Oxidation of Coal Nitrogen," showed that a major fraction (approximately 70%) of NO formed in pulverized coal combustion is due to oxidation of volatile fuel nitrogen, i.e., mechanism 2 above.
One method that is commonly applied to reduce NO formation in pulverized coal firing is the use of low-NOx burners. Low-NOx burners reduce NO formation by delaying the mixing of secondary air into the primary flame. Delay of secondary air mixing produces a lower air/fuel ratio (i.e., air/volatiles ratio) in the primary flame, thus reducing the amount of NO formed from volatile fuel nitrogen. The fact that lowering the air/fuel ratio in the primary flame reduces NO formation is demonstrated by the work of Kawamura and Frey, "Current Developments in Low-Nox Firing Systems," in which the primary air was lowered, causing a reduction in NO formation.
The low-NOx burner principle is less effective on coals of lower volatiles contents and lower heating value of the volatile matter due to the greater difficulty of lowering the air/fuel ratio in the primary flame while preserving ignition stability. This problem in applying low-NOx burners to coals of lower volatiles contents has three components:
(1) As the volatiles content of the coal is reduced, the effective air/fuel ratio in the primary flame (i.e., air/volatiles ratio) increases, causing an increase in NO formation. Although the primary air could, in principle, be decreased for a lower-volatile coal, there is in reality a practical minimum necessary to preserve the transport and drying functions of the primary air.
(2) At sufficiently low heat content of the volatile matter in the coal, flame lift off will occur. This permits mixing of secondary air into the flame prior to complete devolatilization due to the displacement of the ignition zone farther from the burner and thus further increases NO formation. The increase of NO formation due to a lifted flame was documented by Heap et. al. in "Burner Design Principles for Minimum NOx Emissions."
(3) At a still lower heat content of the volatile matter, ignition will become too unstable for practical operation of the burner. This problem is not specific to low-NOx burners. Therefore, the invention is extended to include conventional pulverized coal burners.